Mast cells play an important role in many inflammatory and immunological reactions by releasing an array of mediators. The goal of our studies is to understand the intracellular signal transduction pathways that lead to the release of these molecules. In previous studies we observed that protein tyrosine phosphorylation is an early and critical signal for FceRI induced degranulation. The protein tyrosine kinase Syk was found to be tyrosine phosphorylated and activated after receptor aggregation. Syk was also shown to be essential for the receptor-induced release of inflammatory mediators. Recent studies demonstrated that the adaptor protein Gab2 was found to associate with several signaling molecules including protein tyrosine kinases and phosphatases. Such interactions were shown to be important for Gab2 to function as a regulator of signaling pathways in cells. Studies also defined the mechanisms for the down regulation of signaling molecules that occurs due to a process called ubiquitination whereby molecules are tagged by a covalently coupled protein that targets them for degradation.